1. Field of the Invention
The present invention relates to a printing apparatus which forms a toner image on a latent image carrier, and transfers the toner image on a sheet, yielding a printout, and, more particularly, to a compact printing apparatus.
2. Description of the Related Art
Due to the recent trend to the downsizing and personal usage of computers, it is also desirable that printing apparatuses, which are used as output devices for personal computers or the like, be designed compact. Particularly, there is a demand for A4-size and slim printing apparatuses to match the reduced sizes of personal computers, word processors, etc. Such printing apparatuses are accomplished in wire dot printers and thermal transfer printers whose printing process is easy.
As those wire dot printers, etc. have a low printing resolution, there is a demand for compact and slim electrophotographic recording type printers which can provide high printing resolution.
FIG. 1 is an explanatory diagram of a conventional electrophotographic printer. In the electrophotographic printer, as shown in FIG. 1, a pre-charger 91 for uniformly charging the surface of the photosensitive drum 90, a laser optical system 92 for performing image exposure, a developing unit 93, such as a two-component developing unit, magnetic one-component developing unit or non-magnetic one-component developing unit, a transfer roller 94 for transferring a toner image on the photosensitive drum 90 onto a paper, a cleaner 95 such as a fur brush cleaner or a blade cleaner, and a deelectrifying lamp 96 are disposed around a photosensitive drum 90, such as an organic photosensitive body, Se photosensitive body or a-Si photosensitive body. A thermal fixing unit 97 for fixing the toner image on the sheet with heat or pressure is further provided on a sheet-conveying passage where the sheet is conveyed.
The image forming operation is performed in the following manner. First, the surface of the photosensitive drum 90 is uniformly charged by the pre-charger 91 and then the charged surface is exposed with an optical image corresponding to a target image by the laser optical system 92, thus forming an electrostatic latent image corresponding to the target image on the photosensitive drum 90. Then, charged toners are supplied to the photosensitive drum 90 to develop the electrostatic latent image on the drum 90 in the developing unit 93. The transfer roller 94 as a transfer unit, is disposed in the vicinity of the photosensitive drum 90 with the sheet in between, and electrostatically transfers the toner image on the photosensitive drum 90 onto the sheet. While the sheet carrying this toner image passes through the thermal fixing unit 97, the toner image is fixed on the sheet with heat and pressure, completing the printing. The sheet is then discharged. The top surface of the photosensitive drum 90 after the toner image transfer on the sheet is cleaned with the cleaner 95 to remove the residual toners on the drum 90. Then, the residual charges on the photosensitive drum 90 are removed by the deelectrifying lamp 96 so that the drum 90 returns to the initial state to be ready for another printing operation.
As it is inconvenient to set sheets one by one in such a printing apparatus, it is necessary to provide a hopper retaining a number of sheets. FIGS. 2A through 2D illustrate known arrangements of an electrophotographic printer equipped with conventional hoppers. The first type, as shown in FIG. 2A, has a hopper 8 located at the bottom of the printer and a feeding path for feeding sheets upward from the hopper 8. The print process unit is provided above the hopper 8. A toner image on the photosensitive drum 90 of the print process unit is transferred on a sheet, fed out through the vertical feeding path, and is then fixed by the fixing unit 97 before the sheet is discharged upward of the printer.
The second type has a straight sheet feeding path and the hopper 8 and the print process unit provided on a straight line, as shown in FIG. 2B. A toner image on the photosensitive drum 90 is transferred on a sheet, fed out from the hopper 8 located on the left-hand side in the diagram. The toner image is then fixed by the fixing unit 97 and the sheet is discharged rightward in the diagram.
The third type has an S-shaped sheet feeding path along which a sheet fed out from the hopper located at the bottom of the printer is conveyed, as shown in FIG. 2C. While the sheet is conveyed, a toner image on the photosensitive drum 90 of the print process unit is transferred on the sheet and is then fixed by the fixing unit 97. The sheet is then discharged upward.
As shown in FIG. 2D, the fourth type is similar to the second type shown in FIG. 2B with a difference lying in that the sheet discharging direction is upward in the fourth type. Also known is a modification of the structure shown in FIG. 2B in which printed sheets are returned to the hopper 8, allowing the top of the hopper 8 to be used as a stacker. Such a modification is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 28395/83 and 171181/85.
As the conventional structure shown in FIGS. 2A and 2C have the hopper 8 disposed inside the printer, the printers need a small occupying area. But those printers stand tall and cannot be designed slim. While the conventional structures shown in FIGS. 2B and 2D stand short and are thus slim, they need a larger occupying area.